1. Field of the Invention
The present invention relates to a system for foreseeing deterioration in the interrupting performance of a vacuum interrupter which open and closes electric lines of an electric power system and more particularly to a system for foreseeing deterioration in interrupting performance by measuring a potential difference between a system potential member and an arc shield.
2. Description of the Prior Art
Vacuum interrupters generally do not require maintenance on inspection and have a long service life electrically and mechanically. However, there is a possibility of their being deteriorated in interrupting performance and no longer functioning as an interrupter due to abrasion of electrode contacts or lowering in the degree of vacuum of a vacuum switch tube (current breaking portion) with increase in the number of times of interruption. Therefore, a vacuum interrupter is required to be inspected periodically for its interrupting performance throughout its service life. It has been desired to develop a system capable of accurately foreseeing deterioration in the interrupting performance of a vacuum interrupter in a charged condition.
Heretofore, as a system for foreseeing deterioration in interrupting performance of a vacuum interrupter, there has been known a system which detects lowering in the degree of vacuum of a vacuum switch tube. FIG. 1 is a circuit diagram of a conventional system for detecting a reduction in the degree of vacuum of a vacuum switch tube set forth, for example, in Japanese Patent Laid-Open No. 263015/86. In the figure, numeral 11 denotes a vacuum interrupter having a metallic arc shield 16 insulated from a fixed electrode 12 and a movable electrode 13.
The system for detecting reduction in the degree of vacuum of the vacuum interrupter is provided with a first detector 18 constituted by a series circuit of impedance elements Z.sub.1 and Z.sub.2 as a voltage divider for detecting the waveform of a voltage E.sub.1 to ground of the fixed electrode 12; a second detector 17 constituted by a series circuit of capacitors C.sub.1 and C.sub.2 as a voltage divider for detecting the waveform of a voltage E.sub.3 to ground of the shield 16; a digital or analog phase shift detecting circuit 19 for detecting that a change in phase shift of the waveform of a detected voltage e.sub.3 detected by the second detector 17 has become a value above a predetermined angle, using the waveform of a detected voltage e.sub.1 detected by the first detector 18 as a comparison reference input; a memory circuit 20 for storing the detected output of the detecting circuit 19; and an alarm 21 for issuing a vacuum lowering alarm simultaneously with the storage operation of the memory circuit 20.
Operation will now be described.
As long as the fixed electrode 12 is in a charged condition, regardless of whether the fixed electrode 12 and the movable electrode 13 are in an open or closed state, there appears a voltage E.sub.3 to ground on the shield 16 and there is obtained a waveform of detected voltage e.sub.3 in the detector 17 together with a waveform of detected voltage e.sub.1 in the detector 18. These waveforms of detected voltages e.sub.1, e.sub.3 of the detectors 18 and 17 are analogous to the voltage waveform of the system line and vary at the same rate with variation of line voltage and the occurrence of heavy high-frequency noise.
Thus, a detected voltage e.sub.1 is used as a reference in judging reduction in the degree of vacuum from a change in the detected waveform provided by the detecting circuit 19, whereby there is effected detection without being influenced by voltage variation and noise.
The detecting circuit 19 makes judgment on the basis of an amount of change in phase shift between detected voltages e.sub.1 and e.sub.3 and thereby detects lowering in the degree of vacuum before the lowering progresses up to a degree of making the interrupter inoperative.
Then, the detector output from the detecting circuit 19 is stored in the memory circuit 20, whereby the detected signal indicative of the lowering in the degree of vacuum is maintained to let the alarm 21 continue its warning operation even when the degree of vacuum of the vacuum interrupter lowers rapidly to the level of atmospheric pressure.
Since the conventional system for foreseeing deterioration in interrupting performance of a vacuum interrupter is constructed as above, it inevitably relies on a vacuum lowering detector which detects lowering in the degree of vacuum of the interior of the vacuum switch. Thus, the deterioration of interrupting performance can be detected only in an indirect manner; in other words, the detection is inaccurate. This problem has been involved in the conventional system.